In carbureted V-8 engines, a popular type of intake manifold for racing applications is the tunnel ram. A tunnel ram intake manifold generally features a single large plenum to which eight intake runners connect to the cylinder heads through a long straight path. Usually two 4-venturi, or barrel, carburetors are mounted to the top of the plenum. This manifold and carburetion combination is excellent for maximum power production at high RPM, which is what is needed for racing engines. The length of the intake runners on a tunnel ram intake result in a distinctive tall intake manifold that usually extends through the hood of the vehicle or requires the use of a large hood scoop.
Many automotive enthusiasts build vehicles (cars or trucks) for street use but want them to have the appearance of a racing vehicle. These vehicles are often referred to as “street rods.” Given its distinctive appearance, many street rod enthusiasts would like to use the tunnel ram intake for its race car like appearance on their otherwise street tuned engine. However, the tunnel ram does not work well on most street tuned engines.
In order for a tunnel ram to work properly, the rest of the engine and drive train should be specified for use with a tunnel ram intake manifold. For example, for a tunnel ram to work well, the engine needs to be operated at a relatively high RPM, in the range of approximately 3500-7000 RPM. For operation on the street, particularly with unmodified automatic transmissions, low RPM performance below 3500 RPM is important. In street driving with OEM automatic transmission torque converter stall speeds, and gearing designed to keep RPM down for quiet operation and good fuel economy, the engine is nearly always operating below 3500 RPM. Tunnel ram engines typically do not produce good torque at low RPM. Thus, for street use in slow speed stop and go traffic a tunnel ram equipped engine has poor drivability characteristics. There is no solution currently available which makes a tunnel ram intake manifold suitable for low-RPM operation in a street application. In all cases, existing tunnel ram manifolds have unacceptable reductions in low-RPM torque production.
Also, where the engine is unmodified or only slightly modified, the engine will not be able to operate at the higher RPMs required for the tunnel ram to reach its maximum potential performance benefits. Many OEM camshafts are designed to produce good torque from idle to roughly 5000 RPM. As mentioned above the tunnel ram is designed to work up to 7000 RPM. Thus, with an OEM configured engine a large portion of the tunnel rams effective range is not used.
When used with electronic port fuel injection systems tunnel ram intake manifolds work reasonably well in low-RPM street applications. With fuel injection, a separate fuel injector is provided for each intake runner. This eliminates the poor fuel distribution problems associated with carburetor tunnel ram combinations. In addition, the electronic computer calculates the fuel delivery needed under all conditions, using electronic sensors. However, electronic fuel injection has some important drawbacks for street rod use. First, it is expensive in comparison with a carburetion system. Secondly, the fuel injectors and their associated fuel rails dramatically change the appearance of the intake manifold system, and thus detract from the desired street rod appearance. The desire is typically to have a tunnel ram with two 4-venturi carburetors. Fuel injectors detract from the authentic carbureted racing engine appearance desired by street rod enthusiasts. Another drawback is that tuning electronic fuel injection in any non-OEM application requires knowledge, skills, and equipment that many enthusiasts do not usually have available.
Accordingly, there is a need for a tunnel ram intake manifold that works well with carburetors at lower engine operating speeds. Furthermore, there is a need for a device that allows a traditional tunnel ram intake manifold to operate at lower engine speeds.